El minimalismo en la obra de Sol LeWitt

To be submitted to the American Journal of Veterinary Research

Ray J. Lee, BS; Jennifer A. Schleining, DVM, MS; Paul J. Plummer, DVM, PhD; and Jan A. Shearer, DVM, MS

From the Department of Veterinary Diagnostic and Production Animal Medicine, College of Veterinary Medicine, Iowa State University, Ames, IA 50011

Abstract

Objective – To evaluate the relationship between the mobility score of market weight cattle and the biomechanical properties of the lamina.

Design - Case-control study

Animals – 85 cattle of market weight exhibiting different scores of mobility as described by the North American Meat Institute (NAMI)

Procedures – Animals were observed, mobility scored, and marked using a color code during antemortem exam at a packing plant. Both front hooves were collected at harvest and frozen for transportation to the research facility. Using a randomization chart, left or right hooves were selected and biomechanically tested using a test frame. The test results were then compared between mobility scores.

Results – The laminar strain at break values were significantly increased amongst biomechanical specimens produced from cattle with mobility scores 3 and 4, and the modulus of elasticity values were significantly less for the animals with the most reduced mobility. Peak stress was not significantly altered.

and modulus of elasticity and altered mobility. The results of this study suggest the need for further research of the relationship between mobility and biomechanical properties of bovine lamina.

Introduction

The bovine hoof is an intricate and functional structure that has evolved to help cattle thrive in a multitude of environments. Despite this resiliency, cattle experience a relatively high prevalence of lameness [1]. Lameness is an economic drain on producers [2], decreases the reproductive capacity of cattle [3,4], and is a welfare issue [5]. There is a high prevalence of lameness, as reflected in the fact that lameness has been shown to be one of the leading causes of culling in a dairy environment [6]. Prevention of pain is necessary to provide an adequate level of comfort and is a key component to providing an animal with proper

wellbeing. Prevention of lameness is less costly than treatment and the loss of production the condition incurs [7,8].

There are many tissues and structures within the hoof that can be compromised because of a lesion, a systemic insult, or trauma. One specific example of a structure that is especially sensitive to insult is the lamina [9]. To better understand the physiologic and structural causes of lameness, it is crucial to understand the properties of healthy lamina and compare these properties to lamina from animals with compromised mobility. The hoof of the equine has a similar structure that has been more widely researched than the bovine equivalent to date [10], including finite element modelling [11], which is a mathematical model to describe the interaction of the structures within the hoof that allows the use of theoretical tests rather than attempting to collect data on an actual hoof that may not be possible for practicable or ethical reasons.

Lameness has many causes including genetics [12], housing type, age and parity of the animal [13], and pathologic conditions [14]. Due to the complex innervation of the tissues and structures that the hoof is comprised of, it can be assumed that hoof lesions could cause a significant amount of pain. A better understanding of the pathogenesis of lameness can guide industry to establish appropriate parameters that the tissues and structures can withstand; for instance, guiding flooring types, trailering methods, and handling methods, based on the forces that the hoof capsule can endure.

There are many bone and soft tissue interfaces within the structure of the hoof that can be compromised because of a systemic or traumatic insult resulting in a variety of

lesions. Of specific interest to this study is the laminar junction [15]. The lamina functions as an interface between the hoof wall and the distal phalanx. The hoof wall continually grows from the coronary band to the distal aspect of the hoof and is constantly being remodeled as the hoof wall descends. Due to this complex interface of the tissues comprising the lamina it is crucial to understand its normal properties in addition to examining the properties and conditions of diseased lamina.

Limiting the focus to the dorsal hoof wall, the lamina is comprised of several tissues including the epidermal or insensitive lamina, dermal or sensitive lamina, corium, and basement membrane. The epidermal lamina is the most interior layer of the epidermal layer of the hoof and the most superficial layer of the laminar junction. This portion of the lamina consists of leaflets that interdigitate with the corresponding structure of the sensitive lamina which are extensions of the corium. The interface of these leaflets allows for the continuous renewal of the hoof from the coronary band to the weight bearing surface. The corium is

membrane is the final connection to the bone of the third phalanx.

Another significant purpose of the lamina is its importance in neutralizing the ground reaction force within the hoof capsule by effectively suspending the third phalanx in the hoof capsule. A lack of laminar integrity is linked to a variety of hoof pathologies in cattle

suggesting the distal phalanx may be displaced distally when the strength of the laminar junction is compromised.

The purpose of the research conducted in this study was to investigate the strength of the laminar junction across all four NAMI mobility scores. This consisted of placing a core of hoof tissue under increasing tensile strain until failure and recording the parameters under which it failed. The goal was to examine the biomechanical properties of bovine lamina in states of altered mobility to gain insight into the relationship between mobility and the biomechanical properties of the lamina. It was hypothesized that the animals with higher mobility scores would have lamina that exhibited decreased peak stress, increased strain at break, and decreased modulus of elasticity due to a compromise of the laminar junction.

Materials and Methods

Cattle

Cattle used for this study were of beef-type, without signs of blindness, respiratory disease, or neurologic disturbances. The animals were observed during antemortem

examination at a packing plant, were scored based on mobility and were marked with a color- code during the antemortem exam using a food-safe, water-based livestock marking paint. Animals were re-marked on the left shoulder in the alley with a brightly colored spray chalk product for easier animal identification by the personnel within the plant responsible for collecting hooves. Hooves from 85 animals were collected. This included 24 animals in

mobility score 1, 24 animals in mobility score 2, 17 animals in mobility score 3, and 20 animals in mobility score 4. Forty-five were steers and 40 were heifers.

Table 4-1 Table describing the distribution of cattle by mobility score and sex

Mobility Score Heifer Steer Total

1 15 9 24

2 14 10 24

3 6 11 17

4 5 15 20

Cattle were scored using the North American Meat Institute (NAMI) mobility scoring system. Using this system, a score of “1” represents a normal animal that is moving freely without pain, a mobility score of “2” represents an animal that lags slightly behind the herd as the animals are moved in front of the observer, a score of “3” represents an animal that is lagging significantly and will only move when encouraged to do so by the handler, and a mobility score of “4” is awarded to an animal reluctant to move, and that a handler has significant difficulty provoking the animal to move; mobility score “4” animals are described as ‘statue-like’[16].

Front hooves were collected and immediately assigned a specimen identification number. They were immediately placed into bags labeled with the specimen identification number and transferred to a cooler with dry ice. At the end of the shift, coolers were placed into the blast freezers at the packing plant. Hooves were transported to the research facility packed with dry ice and stored at -20°F until processing.

The lateral claw from the left or right front foot from each animal was selected using a randomization chart to be processed into biomechanical specimens and these specimens were prepared from frozen hooves. Briefly, hooves were sectioned using a reciprocating saw

to fit into a drill press vise. A measurement was made so the specimen was produced from the dorsal hoof wall equidistant from the coronary band to the solar surface of the hoof. The sectioned foot was then placed in a vise attached to the work table of a drill press so that the

dorsal hoof wall was perpendicular to the axis of the spindle of the drill press. A 14 mm arbor-type hole saw with approximately 3 degrees of positive tooth rake without a pilot bit was mounted in the chuck. The hole saw and vise used for this research is shown in Figure 4-

1. The spindle speed of the drill press was 575 revolutions per minute, resulting in a cutting tool speed of 25.9 meters per minute. The feed rate of the hole saw into the hoof was

approximately 0.043mm per revolution or approximately 25mm per minute.

Specimens were processed in a frozen state to prevent tissue damage secondary to the torque applied to the tissues as the cut was made. Specimens were placed inside a labeled conical-bottomed tube, returned to the freezer, and kept frozen until thawed for

biomechanical testing. This method produced specimens 10mm in diameter consisting of a portion of the hoof wall, lamina, corium, and distal phalanx (bone), as seen in Figure 4-2.

Figure 4-2 Photograph and illustration of prepared specimen

Testing was performed with a test frame (Instron Model 4502, Norwood,

Massachusetts) equipped with a 1kN load cell, seen in Figure 4-3; tests were performed in tension with the moving crosshead placed below the specimen.

Figure 4-3 Test Frame: Instron 4502

Specimens were affixed to the grips by set screws that were placed through the grip into either hoof wall or a portion of the distal phalanx.

For testing, the biomechanical specimens were removed from the freezer and allowed to thaw by resting at room temperature for approximately 25 minutes while contained in labeled conical-bottomed tubes. After thawing, specimens were removed from the labeled tubes, and measurements were taken of the laminar thickness, the diameter of the laminar portion of the specimen, and the surface temperature.

The specimen was then secured in custom grips with one grip affixed to a 1kN load cell, the other to the moving crosshead, shown in Figure 4-4.

Figure 4-4 Photograph and illustration of grips demonstrating specimen insertion

Time from removal of specimens from the freezer to insertion into the grips for biomechanical testing averaged 35 minutes. Specimens were trimmed when inserted into the grips if there was extra material protruding beyond what was necessary for fastening the specimen within the grips. Specimens were inserted into the bore of the grip manually and set screws were tightened manually using a hex key. Cross pins attached the grips to the moving cross-head and the load cell, and specimens were placed in tension during a single cycle to failure at a speed of 25mm per minute. The testing procedure for each specimen was approximately four minutes.

Software measured the force transferred through the specimen and displacement of the crosshead. Diameter and laminar thickness of each specimen was entered manually into the program. The software then calculated stress, strain, modulus of elasticity, strain at break, and the yield point of each specimen. These values were exported into a spreadsheet for further evaluation.

The tensile test results from specimens produced from hooves of cattle that were observed to have each mobility score were compared to the tensile test results of the other

comparisons test with significance set at p < 0.05.

Results

All specimens failed at the junction of the dermal and epidermal lamina. Summarized test results for biomechanical specimens for each mobility score can be found in Table 4-1, and graphs depicting the test results can be found in Figure 4-5.

Biomechanical specimens were prepared from a total of 85 cattle hooves. Refer to Table 4-1 for the distribution of animals within each mobility score. There was a downward trend for mean peak stress as mobility score increased, but this was not found to be

statistically significant.

The mean values for peak stress for mobility score 1 was 3.51MPa, mobility score 2 was 3.29MPa, mobility score 3 was 2.98MPa, and mobility score 4 was 2.59MPa. There was no statistical significance between groups (ANOVA p-value: 0.5091), Table 4-3.

The strain at break had significant differences between the mobility scores; ANOVA analysis was used to determine overall significance (p value = 0.0001). The mean strain at break for mobility score 1 was 97.13%, mobility score 2 was 112.79%, mobility score 3 was 178.09%, and mobility score 4 was 170.12%. Significance was found between specimens collected from mobility scores 1 and 2 when compared to mobility scores 3 and 4. However, mobility scores 1 and 2 were not statistically significant nor were mobility scores 3 and 4. Table 4-4.

The median modulus of elasticity for mobility score 1 was 18.60MPa, mobility score 2 was 16.67 MPa, mobility score 3 was 5.98 MPa, and mobility score 4 was 6.93 MPa. There was statistical significance within the group as evidenced by ANOVA (p value = 0.0043). The modulus of elasticity was statistically different between mobility score 1 and 4, and

mobility score 2 and 4. There was no statistical difference between the modulus of elasticity of mobility score 1 and 2, or between mobility score 2 and 3, Table 4-5.

Table 4-2 Summary of biomechanical test results from specimens prepared from hooves of animals that were mobility scored during antemortem examination

Figure 4-5 Box and whisker plots summarizing and comparing biomechanical test results from specimens prepared from hooves of animals that were mobility scored during

antemortem examination. a-b _{Mobility scores with different letters are significantly (p < 0.05)}

Table 4-3 Tukey multiple comparison test summary with p-values for Peak Stress

Mobility Score Median Difference (MPa) p-value 1 vs 2 0.65 0.8999 1 vs 3 0.60 0.8425 1 vs 4 0.95 0.4733 2 vs 3 -0.05 0.8999 2 vs 4 0.30 0.6677 3 vs 4 0.35 0.8999

Table 4-4 Tukey multiple comparison test summary with p-values for Strain at Break

Mobility Score Median Difference (% elongation) p-value 1 vs 2 2.35 0.8008 1 vs 3 -75.52 0.0010 1 vs 4 -61.35 0.0014 2 vs 3 -77.87 0.0081 2 vs 4 -63.70 0.0176 3 vs 4 14.16 0.8999

Mobility Score Median Difference (MPa) p-value 1 vs 2 1.88 0.8999 1 vs 3 12.62 0.1467 1 vs 4 11.67 0.0325 2 vs 3 10.74 0.0791 2 vs 4 12.72 0.0143 3 vs 4 -0.95 0.8999 Discussion

The maximum amount of load transferred through the specimen divided by the cross- sectional area of the specimen is referred to as peak stress. This value represents the

maximum amount of force that a specimen can withstand after it has stretched beyond its ability to return to its original form. While there was an observed downward trend in the peak stress of these specimens as mobility score increased, the difference was not deemed

significant.

Strain at break is defined as the maximum amount the material elongates prior to failure divided by the original length of the portion of interest within the specimen. The significant difference between strain at break in animals with higher mobility scores as compared to those with lower mobility scores suggests that a structural change has occurred within the junction of the lamina for those animals with higher mobility scores resulting in decreased laminar integrity. This alteration in the biomechanical properties of animals with

lower mobility scores could possibly contribute to the decreased mobility of these animals although the exact cause of this is yet to be determined.

Modulus of elasticity describes the stiffness of a material and is determined by the slope of the line within the linear portion of the stress strain curve while the material is behaving elastically and is calculated as stress divided by strain. There was a statistically significant difference found in the modulus of elasticity between the specimens obtained from mobility scores 1 and 4, and 2 and 4. Lamina from cattle with higher mobility scores had a significantly lower modulus of elasticity than cattle with normal mobility and mildly altered mobility. This finding supports our hypothesis that the modulus of elasticity in mobility impaired animals would be less than animals with normal mobility. This also supports the strain at break results and, when interpreted together, indicates that the laminar junction has higher elastic properties in cattle with high mobility scores than normal cattle. This translates into mobility impaired cattle having decreased integrity of the laminar junction which leads to increased likelihood of displacement of the third phalanx within the hoof capsule, decreased ability to dissipate ground reaction forces normally due to altered properties of the lamina, and pain arising from a compromised laminar junction. The exact mechanism, or multifactorial mechanisms, that causes this increased elasticity was not identified in this study and should be the direction of further research.

Limitations of the study would include having a mixed gender population. It is unknown whether hooves obtained from steers and heifers contain the same biomechanical properties. Additionally, variation may exist within the biomechanical properties of the bovine hoof based upon breed and genetic differences; although all animals used for this research were of beef type, specific breeds and crossbreeds were not selected. All cattle for

approximately 300 miles. Nutrition prior to slaughter was also unknown and could influence hoof quality.

In conclusion, the results of this study suggest that in cattle with high mobility scores as assessed with the NAMI scoring system the integrity and the elastic properties of the laminar junction are compromised. There is a relationship between strain at break and modulus of elasticity with the mobility score of finished cattle. As the mobility score of cattle increases the laminar junction becomes more elastic which could explain the degree of discomfort observed in cattle with high mobility scores. Further research on the relationship between mobility score and the biomechanical properties of bovine lamina is warranted.

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